176 UNITY AND DIVERSITY IN BIOCHEMISTRY 



Nevertheless, if we consider the two sets of reactions, A and B, below, we 

 see that either of them is faster than that written above. Both introduce 

 CoA, which acts as the catalyst. 



A) CHaCO'-P + PnA-^SH^CoA— S^COCH. -4-P ^ 



CoA — S-COCHa + NH2— R -^ CH3CO— NHR -j- (CoA-SH) 



B) ATP + CoA -^SH ^ CoA-S-PP + AMP ^X^ 



CoA— S-PP + CH3COOH ^ CoA — S-COCH3 + PP \ 

 C0A-S-COCH3 + NH2-R -^ CH3CO-NHR + (CoA— SH) 



We see that at the end of the reaction, the catalyst remains intact and 

 ready to function again. 



Besides DPN, a number of other coenzymes of the oxido-reduction type 

 act by reason of their ability to be alternatively oxidized and reduced. 

 These are the coenzymes of hydrogen transport : TPN, FMN, FAD and 

 thioctic acid, whose structure we have already considered. 



The few examples above give some idea of the variety of mechanisms 

 of coenzyme action, an action which operates in conjunction with the 

 activation by the enzyme macromolecule itself. The mechanism of this 

 activation is one of the most important problems facing modern bio- 

 chemistry. 



REFERENCES 



Laidler, K. J. (1954). Introduction to the Chemistry of Enzymes, McGraw Hill 



New York. 

 HoFFMANN-OsTENHOF, O. (1954). Enzymologie, Springer, Wien. 

 Mehler, a. H. (1957). Introduction to Enzymology, Academic Press, New York. 

 DixoN, M. and Webb, E. C. (1958). Enzymes, Longmans, Green and Co, London. 



